This invention relates to laminar flow elements as employed in thermal mass flow meters, and in particular to a laminar flow element that is adjustable over an extended fluid flow range.
Thermal mass flow meters are essential monitoring devices for many laboratory and commercial applications. They are routinely employed in semiconductor equipment fabrication, vacuum processes such as vapor deposition, and petrochemical, pharmaceutical, and food processing industries. Their main use is in metering gas flow, and controlling gas flow when used in conjunction with a mass flow controller. In general there are two main methods for metering gas flow; volumetric, and mass based. A problem with volumetric metering is that this type of measurement is dependent on knowing the temperature and pressure of the gas for accurate results. Obviously using volumetric metering operators must recalibrate their equipment for each condition that applies to the gas metering being performed. With a mass flow based system on the other hand, the sensor within the thermal mass flow meter is not significantly effected by changes in temperature and pressure. This is because sensing of gas flow is done by a mass of gas as it passes through a sensing conduit. This is called "Pound Molecular Weight Flow". The process can be envisioned as each molecule of the gas carrying an amount of heat from point A in a sensor conduit to point B in that conduit, with temperature and pressure variations playing an insignificant role.
In a typical thermal mass flow meter incoming gas is split into two streams by an appropriately selected restriction device often referred to as a laminar flow element, or a restrictor flow element. A fixed portion of the gas stream is thus forced to flow through a by-pass conduit, which is the sensor tube for the gas flow. The major portion of the gas stream flows through the length of the other conduit containing the laminar flow element.
The basic concept of the thermal mass flow meter is that if the gas flow is laminar in both the sensor tube and conduit containing the laminar flow element, then the ratio of the flow detected through the sensor tube (Qs), and the flow rate through conduit containing the laminar flow element (Qm) are proportional to one another according to the following equation: ##EQU1##
In U.S. Pat. No. 4,843,881, issued Jul. 4, 1989, a mass flow sensing system suitable for making use of the instant invention is described, the disclosure of which is hereby incorporated by reference. U.S. Pat. No. 4,843,881 teaches the use of two heated resistor windings on the outer surface of a thin walled, stainless steel sensor tube to impart heat to the gas medium flowing through the sensor tube. When no flow is taking place both windings are kept at the same temperature. During gas flow heat is carried from the winding near the gas flow inlet to a winding further along the length of the sensor tube in the direction of the gas flow outlet, which results in proportional changes in the temperature of the windings. These changes in temperature of the windings results in changes of resistance values of the windings. Each winding is part of a Wheatstone bridge. Instantaneous resistance values are detected by the Wheatstone bridges, and by means of an amplifier circuit an analog output signal of 0 to 5 Vdc (volts, direct current) or 4 to 20 ma (milliamps) is obtained. The sensor tube is appropriately calibrated to indicate the flow rate of the combined gas flow rates taking place in both the sensor tube and in the conduit containing the laminar flow element.
As can be seen from the above description the concept of laminar flow is essential to understanding the functioning of a thermal mass flow meter. Laminar flow is layered, orderly, non-turbulent flow. Laminar flow is defined by a dimensionless number called the "Reynolds Number" which is expressed by the following formula: ##EQU2## where:
p=density of gas (Gm/cc)
D=inside diameter of conduit (mm)
V=velocity of gas flow (mm/sec)
u=viscosity of gas (centipoise)
Fluid flow characterized by a Reynolds Number of less than 2,000 is considered laminar fluid flow.
In the current state of the art in thermal mass flow meter designs laminarity of fluid flow is obtained by the sensor tube being a long, thin walled, capillary tube, with the ratio of length to internal diameter of the capillary tube generally being of the order of 25 to 1. The major portion of the fluid flows through a conduit containing a laminar flow element carefully designed to both restrict fluid flow, and to do so in a manner that most favors the creation of a laminar fluid flow. A wide variety of laminar fluid flow elements are used with different sizes and even geometric attributes to achieve the desired flow domain for different applications. Currently laminar flow element designs necessitate a large number of different laminar flow elements to be kept on hand, with generally cumbersome resultant calibrating procedures. The instant invention on the other hand provides an adjustable laminar flow element so that optimal laminar flow fluid dynamics are obtained over an extended flow range, and by one single laminar flow element.
It is therefore a primary object of the invention to provide a means for obtaining adjustable laminar fluid flow over an extended fluid flow range.
A further object is to provide a laminar flow element for achieving optimal fluid dynamics.
Another object is to provide a single laminar flow element to be used over extended ranges of fluid flow.
Still another object is to provide a cleanable laminar flow element.
An additional object is to provide a laminar flow element that is less susceptible to contamination.
Another object is to provide a combined laminar flow element and desiccant.
A further object is to provide a combined laminar flow element and anti-static device.
Another object is to provide a laminar flow element that can be calibrated by end users of this equipment.